The present invention relates to soft polyolefin compositions with improved processability. More particularly, the invention relates to soft heterophasic polyolefin compositions with improved processability obtained by addition of certain processing aids.
Soft polyolefin compositions can be obtained by blending a crystalline thermoplastic fraction with an elastomeric fraction. Typically, such compositions are heterophasic since the crystalline fraction constitutes a matrix in which the elastomeric phase is finely dispersed.
Soft heterophasic polyolefin compositions can be obtained by a variety of processes, including sequential polymerisation in multiple reactors producing in-reactor blends.
Soft polyolefin compositions having elastic properties but still retaining a thermoplastic behaviour are used in many applications since they can be transformed into shaped articles by the same fabrication processes used for thermoplastic polymers, including extrusion processes and injection moulding.
Soft polyolefin compositions in which the crystalline fraction comprises a substantial amount of a propylene homopolymer or a random copolymer of propylene with ethylene and or another α-olefin, and the elastomeric fraction comprises a substantial amount of an elastomeric ethylene-propylene copolymer, optionally containing a diene (EPDM), can advantageously be prepared by sequential polymerisation in at least two reactors in the presence of a Ziegler/Natta or a metallocene catalyst system, as described for example in WO 03/011962.
Compositions comprising a substantial amount of an elastomeric fraction having a high intrinsic viscosity, however, may be difficult to process in the molten state due to a very low melt flow rate (MFR), often below 1 (230° C., 2.16 kg, ASTM D 1238, Condition L). Generally, a widely used technique to increase the MFR of a polyolefin material is visbreaking.
The process of visbreaking a polyolefin material is well known to those skilled in the art. Generally, it is carried out by using a peroxide in liquid or powder form or absorbed on a carrier. The polyolefin composition/peroxide mixture is then introduced into a means for thermally plasticizing and conveying the mixture, e.g., an extruder at elevated temperature. Residence time and temperature are controlled in relation to the particular peroxide selected (i.e., based on the half-life of the peroxide at the process temperature of the extruder) so as to effect the desired degree of polymer chain degradation. The net result is to narrow the molecular weight distribution of the polymers present in the composition, as well as to reduce the overall molecular weight and thereby increase the MFR relative to the as-polymerized polymer. For example, a polymer with a fractional MFR (i.e., less than 1), or a polymer with a MFR of 0.5-10, can be selectively visbroken to a MFR of 15-50 by selection of peroxide type, extruder temperature and extruder residence time. Sufficient care should be exercised in the practice of the procedure to avoid cross-linking in the presence of an ethylene-containing copolymer. Peroxide visbreaking, however, results often in sticky materials and in worsening of mechanical properties. Therefore, improved processability obtained by visbreaking suffers the big disadvantage of a lower quality of the polyolefin composition.
Use of melt viscosity reducing agents instead of visbreaking to improve processability of certain polyolefin materials is disclosed in the patent literature.
U.S. Pat. No. 6,277,918 B1 discloses the use of crystalline polybutene-1 with polyolefin compositions comprising a substantial amount (above 75% by weight) of an ethylene copolymer of the LLDPE type.
U.S. Pat. No. 6,667,364 B2 discloses the use of a melt viscosity reducer to improve the processability of PE-rich TPV (“thermoplastic vulcanizates”) in which PE is the continuous hard phase and a particular EPDM rubber is the soft phase. Viscosity reducers disclosed in this patent include isotactic poly(1-butene) and certain oils or waxes. The high viscosity of these compositions is attributable to the compatibility of PE with EPDM rubber. A TPV in which either PP or the rubber is present in a larger amount than the other is deemed to be processable (column 2, lines 51-55).